The present invention relates to an exhaust emission purifying device capable of removing nitrogen oxides (NOx) from the exhaust emitted from an internal combustion engine capable of lean burning.
Recently, as a vehicle-mounted internal combustion engine, a lean-burn type internal combustion engine is being developed which is capable of burning an air-fuel mixture with excessive oxygen. Along with this, a technology for removing harmful gas components, in particular, nitrogen oxides (NOx), contained in the exhaust gas from a lean-burn type internal combustion engine is being developed.
As a technology for purifying the exhaust emitted from a lean-burn type internal combustion engine, a technology is known according to which an NOx absorbing material, such as an occlusion reduction type NOx catalyst, is provided in the exhaust passage of an internal combustion engine.
The NOx absorbing material absorbs the NOx in the exhaust when the air-fuel ratio of the exhaust flowing therethrough indicates excessive oxygen (that is, in the case of a lean air-fuel ratio), and releases the NOx it has absorbed when the oxygen concentration of the exhaust flowing therethrough is reduced. The occlusion reduction type NOx catalyst, which is an example of the NOx absorbing material, is a catalyst which absorbs the NOx in the exhaust when the air-fuel ratio of the exhaust flowing in is lean, and reduces the NOx to nitrogen (N2) while releasing the NOx it has absorbed when the oxygen concentration of the exhaust flowing in is reduced.
When the occlusion reduction type NOx catalyst is arranged in the exhaust passage of a lean-burn-type internal combustion engine, the NOx in the exhaust is absorbed by the occlusion reduction type NOx catalyst when the air-fuel ratio of the exhaust is lean, and the NOx which has been absorbed by the occlusion reduction type NOx catalyst is released as NO2 when the air-fuel ratio of the exhaust is stoichiometric or rich, the NO2 reacting with the reduction components in the exhaust, such as hydrocarbon (HC) and carbon monoxide (CO), to be thereby reduced to nitrogen (N2)
In some cases, the fuel of an internal combustion engine contains sulfur content. If such a fuel is burnt in an internal combustion engine, the sulfur content in the fuel is oxidized to generate sulfur oxides (SOx), such as SO2 and SO3. The occlusion reduction type NOx catalyst absorbs the SOx in the exhaust on the same principle as the absorption of NOx, so that, when the occlusion reduction type NOx catalyst is arranged in the exhaust passage of an internal combustion engine, the occlusion reduction type NOx catalyst absorbs not only NOx but also SOx.
The SOx absorbed by the occlusion reduction type NOx catalyst forms a stable sulfate with passage of time, so that, in the same conditions for effecting release/reduction of NOx from the occlusion reduction type NOx catalyst, it is not easily decomposed or released and tends to be accumulated in the occlusion reduction type NOx catalyst. When the accumulation amount of SOx in the occlusion reduction type NOx catalyst increases, the NOx absorption capacity of the occlusion reduction type NOx catalyst decreases, and it becomes impossible to remove the NOx in the exhaust to a sufficient degree, that is, so-called SOx poisoning occurs.
To cope with this problem, there has conventionally been proposed an exhaust emission purifying device in which an SOx absorbing material for absorbing the SOx contained in the exhaust is provided in the exhaust passage on the upstream side of the occlusion reduction type NOx catalyst. When the air-fuel ratio of the exhaust flowing in is lean, the SOx absorbing material absorbs the SOx in the exhaust, and when the air-fuel ratio of the exhaust flowing in is stoichiometric or rich, it releases the SOx it has absorbed as SO2.
In this exhaust emission purifying device, the SOx in the exhaust is removed on the upstream side of the occlusion reduction type NOx catalyst, and it is possible to prevent the SOx poisoning of the occlusion reduction type NOx catalyst.
However, there is a limitation to the SOx absorption capacity of the SOx absorbing material, so that it is necessary to perform a processing for releasing the SOx absorbed by the SOx absorbing material, that is, a regeneration processing, before the SOx absorption capacity of the SOx absorbing material has been saturated.
An example of the SOx absorbing material regeneration technology is disclosed in Japanese Patent No. 2605580. According to this patent official gazette, to release the SOx absorbed by the SOx absorbing material, it is necessary for the air-fuel ratio of the exhaust flowing in to be stoichiometric or rich. Further, the higher the temperature of the SOx absorbing material, the easier it is for the SOx to be released.
In the exhaust emission purifying device disclosed in the above-mentioned official gazette, to prevent the SOx released from the SOx absorbing material from being absorbed by the occlusion reduction type NOx catalyst, there are provided an bypass path branching off from the exhaust pipe connecting the SOx absorbing material and the occlusion reduction type NOx catalyst and bypassing the occlusion reduction type NOx catalyst, and an exhaust switching valve for selectively switching the exhaust flow between the occlusion reduction type NOx catalyst and the bypass path. When executing the regeneration process of the SOx absorbing material, the exhaust switching valve is controlled so as to cause all the exhaust from the SOx absorbing material to flow through the bypass path.
Further, in the exhaust emission purifying device disclosed in the above-mentioned official gazette, when the regeneration process of the SOx absorbing material is not being performed, in other words, when the absorption or releasing of NOx is to be performed by the occlusion reduction type NOx catalyst, the exhaust switching valve is controlled so as to cause all the exhaust to flow through the occlusion reduction type NOx catalyst.
As is known in the art, the sealing property of the exhaust switching valve used in the above exhaust emission purifying device cannot be regarded as perfect, and the valve allows leakage of approximately 1 to 10% of the exhaust. Thus, in the exhaust emission purifying device disclosed in the above official gazette, if the exhaust switching valve is controlled so as to allow the exhaust to flow through the occlusion reduction type catalyst and as to prevent the exhaust from flowing into the bypass path, some exhaust is allowed to leak through the exhaust switching valve to the bypass path, with the result that the NOx contained in the exhaust leaking through the exhaust switching valve to the bypass path is released to the atmosphere without being removed from the exhaust.
As a result of the recent progress in the catalyst technology, the NOx purifying ratio by the occlusion reduction type NOx catalyst is over 90%. Thus, the deterioration in exhaust emission control due to the leakage through the exhaust switching valve cannot be neglected.
Further, the above-described conventional exhaust emission purifying device for an internal combustion engine is not provided with a means for reducing the hydrocarbon (HC) in the exhaust when the internal combustion engine is started when the ambient temperature is low (that is, at the time of low-temperature startup), so that there is the danger of the hydrocarbon (HC) in the exhaust being released to the atmosphere without being purified from the exhaust. Thus, the conventional exhaust emission purifying device leaves room for improvement.
The present invention has been made in view of the above various problems. It is an object of the present invention to prevent a deterioration in exhaust emission purifying due to leakage of the exhaust into the bypass path when the bypass path is closed by the exhaust flow switching means.
Another object of the present invention is to reduce the hydrocarbon concentration in the exhaust when the internal combustion engine is started at low temperature.
To achieve the above objects, the present invention adopts the following means.
An exhaust emission purifying device of an internal combustion engine according to the present invention comprises a lean-burn-type internal combustion engine capable of burning an air-fuel mixture with excessive oxygen, an NOx absorbing material which is arranged in an exhaust passage of the internal combustion engine and which is adapted to absorb nitrogen oxides (NOx) when the air-fuel ratio of the exhaust flowing-in is lean and to release the nitrogen oxides (NOx) it has absorbed when the oxygen concentration of the exhaust flowing-in is low, a bypass path branching off from a portion of the exhaust passage on the upstream side of the NOx absorbing material and allowing the exhaust to flow so as to bypass the NOx absorbing material, an exhaust flow switching means for selectively switching the exhaust flow between the NOx absorbing material and the bypass path, an SOx absorbing material arranged in the exhaust passage on the upstream side of the exhaust flow switching means and adapted to absorb sulfur oxides (SOx) when the air-fuel ratio of the exhaust flowing in is lean and to release the sulfur oxides (SOx) it has absorbed when the oxygen concentration of the exhaust flowing in is low, and an NOx catalyst provided in the bypass path and adapted to remove the nitrogen oxides (NOx) when the air-fuel ratio of the exhaust is lean.
Normally, when purifying the exhaust discharged from an internal combustion engine and, in particular, when purifying the nitrogen oxides (NOx) contained in the exhaust, the exhaust flow switching means is controlled such that the exhaust discharged from the internal combustion engine passes through the NOx absorbing material. In this case, although no exhaust ought to flow through the bypass path. However, when the sealing property of the exhaust flow switching means is not perfect, a minute amount of exhaust is allowed to leak to the bypass path through the exhaust flow switching means.
In contrast, in the exhaust emission purifying device for an internal combustion engine of the present invention, the minute amount of exhaust leaking to the bypass path passes through the NOx catalyst provided in the bypass path at a very low spatial velocity (hereinafter, spatial velocity will be abbreviated as SV), so that the nitrogen oxides (NOx) contained in the exhaust leaking to the bypass path is efficiently purified by the NOx catalyst.
As a result, in the exhaust emission purifying device for an internal combustion engine of the present invention, even if some exhaust is allowed to flow through the bypass path when no exhaust should flow through the bypass path, the exhaust flowing through the by pass path can be released to the atmosphere after being purified, so that it is advantageously possible to achieve an improvement in reliability in exhaust emission control.
Further, in the exhaust emission purifying device for an internal combustion engine of the present invention, there is provided in the exhaust passage on the upstream side of the exhaust flow switching means an SOx absorbing material adapted to absorb SOx when the air-fuel ratio of the exhaust is lean and to release the SOx it has absorbed when the oxygen concentration of the exhaust flowing in is low, so that the SOx in the exhaust is absorbed by the SOx absorbing material before the exhaust flows to the NOx absorbing material, whereby there is no danger of the NOx absorbing material undergoing SOx poisoning.
In the exhaust emission purifying device for an internal combustion engine of the present invention, examples of an internal combustion engine capable of lean burning include an in-cylinder injection type lean burn gasoline engine and a diesel engine. In the case of a lean burn gasoline engine, the air-fuel ratio of the exhaust can be controlled by controlling the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber. In the case of a diesel engine, the air-fuel ratio of the exhaust can be controlled by performing a secondary fuel injection during intake stroke, expansion stroke, or exhaust stroke, or by supplying a reducing agent to the exhaust passage on the upstream side of the NOx absorbing material. Here, the air-fuel ratio of the exhaust is the ratio of the air to the fuel (hydrocarbon) supplied to the engine intake passage and to the portion of the exhaust passage on the upstream side of the NOx absorbing member.
In the exhaust emission purifying device for an internal combustion engine of the present invention, examples of the NOx absorbing material include an occlusion reduction type NOx catalyst. The occlusion reduction type NOx catalyst is a catalyst which absorbs the nitrogen oxides (NOx) in the exhaust when the air-fuel ratio of the exhaust flowing in is lean, and reduces the nitrogen oxides (NOx) it has absorbed to nitrogen (N2) while releasing the nitrogen oxides when the oxygen concentration of the exhaust flowing in is low.
Examples of an occlusion reduction type NOx catalyst include a catalyst comprising an alumina carrier which carries thereon at least one of the following metals: alkali metals, such as potassium K, sodium Na, lithium Li, and cesium Cs; alkali earth metals, such as barium Ba and calcium Ca; and rare earth metals, such as lanthanum La and yttrium Y, together with a noble metal, such as platinum Pt.
In the exhaust emission purifying device for an internal combustion engine of the present invention, the exhaust flow switching means may be formed by a single switching valve provided in the branching portion of the bypass path, or by providing a first opening/closing valve in the exhaust passage at a position nearer to the NOx absorbing material than to the branching portion and a second opening/closing valve in the bypass path.
In the exhaust emission purifying device for an internal combustion engine of the present invention, examples of the SOx absorbing material include a material comprising a carrier consisting alumina and carrying thereon at least one of the following metals: transition metals such as copper Cu, iron Fe, manganese Mn, and nickel Ni; and sodium Na; titanium Ti; and lithium Li. In order that the SOx may be easily absorbed by the SOx absorbing material in the form of sulphate ions SO42xe2x88x92, it is desirable that one of platinum Pt, palladium Pd, and rhodium Rh be carried on the carrier of the SOx absorbing material.
In the exhaust emission purifying device for an internal combustion engine of the present invention, examples of the NOx catalyst provided in the bypass path include a selective reduction type NOx catalyst which reduces or decomposes the nitrogen oxides (NOx) when hydrocarbon exists in an atmosphere with excessive oxygen. The selective reduction type NOx catalyst generally exhibits its characteristics of providing a high NOx purification ratio even with respect to a small amount of hydrocarbon (HC) when the exhaust flows at a low SV, and is capable of purifying hydrocarbon (HC) or nitrogen oxides at a purifying ratio of 70 to 80%.
Thus, in the case in which the exhaust flow switching means is controlled such that the exhaust emitted from the internal combustion engine flows through the NOx absorbing material, when a minute amount of exhaust leaks to the bypass path from the exhaust flow switching means, the minute amount of exhaust flows through the NOx catalyst at a low SV so that the nitrogen oxides (NOx), hydrocarbon (HC), etc. contained in the exhaust are efficiently purified. Examples of the selective reduction type NOx catalyst include a catalyst comprising a zeolite carrier carrying platinum (Pt) thereon.
The NOx catalyst provided in the bypass path may be an occlusion reduction type NOx catalyst which absorbs nitrogen oxides (NOx) when the air-fuel ratio of the exhaust flowing in is lean and which reduces and purifies the nitrogen oxides (NOx) it has absorbed while releasing the same when the oxygen concentration of the exhaust flowing in is reduced and there exists a reducing agent such as hydrocarbon (HC).
Examples of the occlusion reduction type NOx catalyst include a catalyst comprising the alumina carrier which carries thereon at least one of the following metals: alkali metals, such as potassium K, sodium Na, lithium Li, and cesium Cs; alkali earth metals, such as barium Ba and calcium Ca; and rare earth metals, such as lanthanum La and yttrium Y; together with a noble metal, such as platinum Pt.
In the exhaust emission purifying device for an internal combustion engine of the present invention, it is desirable that the exhaust flow switching means be controlled so as to permit the exhaust to flow to the NOx absorbing material and to inhibit the exhaust to flow to the bypass path when the air-fuel ratio is controlled to be lean, and to permit the exhaust to flow to the bypass path and inhibit the exhaust to flow to the NOx absorbing material when the air-fuel ratio is controlled to be stoichiometric or rich.
When the air-fuel ratio of the exhaust is controlled to be lean, sulfur oxides (SOx) contained in the exhaust are absorbed by the SOx absorbing material, and the exhaust from which the sulfur oxides (SOx) have been removed flows through the NOx absorbing material, so that only the nitrogen oxides (NOx) in the exhaust are absorbed by the NOx absorbing material, and it is possible to reliably prevent occurrence of so-called SOx poisoning, in which sulfur oxides (SOx) are absorbed by the NOx absorbing material.
On the other hand, when the air-fuel ratio of the exhaust is controlled to be stoichiometric or rich, the exhaust passed through the SOx absorbing material is discharged by way of the bypass path, and no exhaust is allowed to flow through the NOx absorbing material, so that, even if sulfur oxides (SOx) are released from the SOx absorbing material, the sulfur oxides (SOx) released from the SOx absorbing material do not flow into the NOx absorbing material, and there is no danger of the NOx absorbing material undergoing SOx poisoning. Since the exhaust having the air-fuel ratio of stoichiometric or rich flows through the NOx catalyst in the bypass path, the sulfuroxides (SOx) adsorbed by the NOx catalyst are released to become SO2.
Here, the expression: xe2x80x9cwhen the air-fuel ratio of the exhaust is controlled to be stoichiometric or richxe2x80x9d is of course the concept which covers the case in which the air-fuel ratio of the exhaust is controlled to be stoichiometric or rich in order to perform the regeneration processing on the SOx absorbing material, but also covers the case in which the air-fuel ratio of the exhaust becomes stoichiometric or rich as a result of the air-fuel ratio of the air-fuel mixture being controlled to be stoichiometric or rich according to the operating condition of the engine.
Examples of the engine operating condition in which the air-fuel ratio of the air-fuel mixture is stoichiometric or rich include high-load operating condition, a full-load operating condition, and warming up operation after engine startup.
In the exhaust emission purifying device for an internal combustion engine of the present invention, the NOx catalyst provided in the bypass path may have a 3-way purifying function and an HC adsorption capacity at a low temperature. In this case, it is desirable for the exhaust flow switching means to be controlled such that when the temperature of the exhaust is less than a predetermined temperature, the exhaust is led to the bypass path and is prevented from flowing into the NOx absorbing material, and that when the temperature of the exhaust is higher than the predetermined temperature, the exhaust is led to the NOx absorbing material and is prevented from passing through the bypass path.
When the temperature of the exhaust is less than the predetermined temperature, the NOx absorbing material is not activated yet, so that it is impossible to sufficiently purified the exhaust at this temperature by passing it through the NOx absorbing material. In the exhaust emission purifying device of the present invention, when the exhaust temperature is less than the predetermined temperature, the exhaust is caused to flow through the bypass path, whereby the hydrocarbon (HC) in the exhaust is adsorbed by the NOx catalyst.
As a result, when the internal combustion engine is started at a low temperature, the exhaust is advantageously released to the atmosphere after being purified.
On the other hand, when the temperature of the exhaust has been raised to a level higher than the predetermined temperature, the NOx absorbing material is activated, and can exert the purifying capacity, so that the exhaust flow switching means is controlled such that the exhaust is led to the NOx absorbing material and that the exhaust is prevented from flowing through the bypass path.
At this time, if the sealing property of the exhaust flow switching means is not perfect, a minute amount of exhaust leaks from the exhaust flow switching means to the bypass path. However, since the amount of exhaust leaking from the exhaust flow switching means to the bypass path is relatively small, the minute amount of exhaust flows through the NOx catalyst provided in the bypass path at a low SV.
When the exhaust flows through the NOx catalyst at a low SV, the reaction of the nitrogen oxides (NOx) contained in the exhaust with the hydrocarbon (HC) adsorbed by the NOx catalyst is promoted, so that the nitrogen oxides (NOx) in the exhaust are effectively purified, and an improvement is achieved in terms of exhaust emission purification. Further, as stated above, the hydrocarbon (HC) adsorbed by the NOx catalyst is consumed as the reducing agent for the nitrogen oxides (NOx), and in addition, it reacts with the oxygen contained in the exhaust to be thereby purified, so that the exhaust emission purification is further improved.
In the exhaust emission purifying device for an internal combustion engine of the present invention, when the internal combustion engine is an in-cylinder injection type internal combustion engine provided with a fuel injection valve for directly injecting fuel into the combustion chamber of the internal combustion engine, and the SOx absorbing material has a 3-way purifying function, the exhaust flow switching means may be controlled at the startup of the internal combustion engine so as to throttle the exhaust flow amount passing through the NOx absorbing material and the NOx catalyst and as to cause the fuel injection valve to perform a secondary fuel injection during the expansion stroke of each cylinder in addition to the injection of the fuel for combustion.
When, at the startup of the internal combustion engine, the exhaust flow passing through the NOx absorbing material and the NOx catalyst is throttled, the back pressure acting on the internal combustion engine rises to cause the temperature of the exhaust to rise. When, in this condition, the fuel is injected secondarily from the fuel injection valve during the expansion stroke of each cylinder, the reaction of the injected fuel with the oxygen in the exhaust is promoted. When the reaction of the fuel with the oxygen is promoted, the quantity of heat generated at the time of the reaction of the fuel and oxygen increases, and the exhaust temperature rises. When the exhaust, which has thus attained high temperature, flows into the SOx absorbing material, the heat of the exhaust is transmitted to the SOx absorbing material, and the temperature of the SOx absorbing material rises abruptly, with the result that the 3-way purifying function of the SOx absorbing material is activated at an early stage. As a result, it is possible to improve the exhaust emission purification when the internal combustion engine is started at a low temperature.
In the exhaust emission purifying device for an internal combustion engine of the present invention, it is also possible to further provide a temperature rise restraining means which controls the exhaust flow switching means such that the exhaust flows through both the NOx absorbing material and the NOx catalyst when the temperature of the NOx absorbing material becomes higher than a predetermined temperature when the exhaust flow switching means is being controlled such that the exhaust is led to the NOx absorbing material and the exhaust is prevented from flowing into the NOx catalyst.
The NOx absorbing material has such characteristics that, when it is in a predetermined temperature range, efficiently absorbs the nitrogen oxides (NOx), so that when the exhaust temperature becomes higher than a predetermined temperature when the entire amount of exhaust is flowing through the NOx absorbing material, the temperature of the NOx absorbing material exceeds the activation temperature range, with the result that it becomes difficult for the NOx absorbing material to absorb nitrogen oxides (NOx). In view of this, the temperature rise restraining means so controls the exhaust flow switching means that the exhaust flows through both the NOx absorbing material and the NOx catalyst.
In this case, the amount of exhaust flowing through the NOx absorbing material is reduced by half as compared with the case in which the entire amount of exhaust flows through the NOx absorbing material, so that the quantity of heat the NOx absorbing material receives from the exhaust is also reduced by half, and there is no excessive temperature rise of the NOx absorbing material, with its temperature being kept within the activation temperature range.
When the entire amount of exhaust is flowing through the NOx absorbing material, the air-fuel ratio of the exhaust is controlled to be lean, so that it is preferable that the NOx catalyst is one which functions to purify the NOx in the exhaust when the air-fuel ratio of the exhaust is lean. Examples of such a NOx catalyst include an occlusion reduction type NOx catalyst.
The temperature rise restraining means may be one which executes the SOx poisoning regeneration processing on the NOx catalyst immediately before controlling the exhaust flow switching means to cause the exhaust to flow through both the NOx absorbing material and the NOx catalyst.
In the exhaust emission purifying device for an internal combustion engine of the present invention, when the air-fuel ratio of the exhaust is being controlled to be stoichiometric or rich, the exhaust flow switching means is controlled such that the entire amount of the exhaust flows through the NOx catalyst, so that, in this process, it is assumed that the sulfur oxides (SOx) released from the SOx absorbing material is adsorbed by the NOx catalyst to thereby cause SOx poisoning. At the same time, when the exhaust flows through both the NOx absorbing material and the NOx catalyst in the condition in which the NOx catalyst has undergone SOx poisoning, it is assumed that the NOx purifying ratio of the NOx catalyst will be reduced.
In the exhaust emission purifying device for an internal combustion engine of the present invention, the exhaust flow switching means may be controlled such that when the internal combustion engine is performing warming-up operation, the exhaust is led to the NOx catalyst and that the exhaust is prevented from flowing into the NOx absorbing material, switching being performed such that after the completion of the warming up of the internal combustion engine, the exhaust is led to the NOx absorbing material and is prevented from flowing into the NOx catalyst at the time when the NOx exhaust amount from the internal combustion engine has become less than the predetermined amount.
When the internal combustion engine is the warming up state, the air-fuel ratio of the exhaust is controlled to be stoichiometric or rich, so that the exhaust flow switching means is controlled so as to cause the entire amount of exhaust to flow through the NOx catalyst to prevent the sulfur oxides (SOx) released from the SOx absorbing material from flowing into the NOx absorbing material. Thus, no exhaust flows through the NOx absorbing material until the warming up of the internal combustion engine is completed and the operating condition of the internal combustion engine has been switched to the lean air-fuel ratio operation, and the NOx absorbing material is expected to be in the non-activated state even after the completion of the warming up of the internal combustion engine. In such a case, when the exhaust flow switching means is controlled such that the entire amount of exhaust flows through the NOx absorbing material, the nitrogen oxides (NOx) in the exhaust are not purified by the NOx absorbing material, and there is the danger of deteriorating the exhaust emission purification.
In contrast, in the exhaust emission purifying device for an internal combustion engine of the present invention, when, after the completion of the warming up of the internal combustion engine, the amount of nitrogen oxides (NOx) discharged from the internal combustion engine becomes less than the predetermined amount, the exhaust flow switching means is switched from the state in which the entire amount of exhaust flows through the NOx catalyst to the state in which the entire amount of exhaust flows through the NOx absorbing material.
In this case, after the completion of the warming up of the internal combustion engine, the exhaust gas flows into an NOx absorbing material in the non-activated state, so that the temperature of the NOx absorbing material rises due to the heat of the exhaust. In this process, the exhaust flows through the NOx absorbing material in the non-activated state, but, since the amount of nitrogen oxides (NOx) contained in the exhaust is very small, it is possible to raise the temperature of the NOx absorbing material while restraining the deterioration in exhaust emission to a minimum.
The amount of nitrogen oxides (NOx) discharged from the internal combustion engine becomes less than the predetermined amount when, for example, the vehicle on which the internal combustion engine is mounted is running at decelerated speed, or when the load of the internal combustion engine becomes less than a predetermined value, and, in this regard, the so-called fuel cut condition is preferable, in which the fuel injection is stopped in the internal combustion engine.
Further, while, at the time of warming up of the internal combustion engine, the air-fuel ratio of the exhaust is being controlled to be stoichiometric or rich, the exhaust flow switching means may be controlled such that the exhaust is led to the Nox catalyst and that the exhaust is prevented from flowing into the NOx absorbing material, and while, at the warming up of the internal combustion engine, the amount of NOx discharged from the internal combustion engine is less than the predetermined amount, it may be controlled such that the exhaust is led to the NOx absorbing material and that the exhaust is prevented from flowing into the NOx catalyst.
In this case, it is possible to activate the NOx absorbing material while restraining the deterioration of exhaust emission at the time of warming up of the internal combustion engine. As a result, it is possible to improve the exhaust emission when the exhaust starts to flow through the NOx absorbing material after the completion of the warming up of the internal combustion engine.
In the exhaust emission purifying device for an internal combustion engine of the present invention, when SOx poisoning of at least one of the NOx absorbing material and the NOx catalyst is detected, it is possible to control the exhaust flow switching means to such that the exhaust flows through both the NOx absorbing material and the NOx catalyst, and to further provide an SOx poisoning regeneration means for simultaneously executing a SOx poisoning regeneration processing on the NOx absorbing material and the NOx catalyst.
When simultaneously performing the SOx poisoning regeneration on the NOx absorbing material and the NOx catalyst, the frequency of execution of the SOx poisoning regeneration processing is reduced as compared with the case in which the SOx poisoning regeneration is individually and separately performed on the NOx absorbing material and the NOx catalyst. In the SOx poisoning regeneration processing, it is necessary to raise the temperature of the NOx absorbing material and the NOx catalyst to a relatively high temperature range, so that the fuel is burnt in the NOx absorbing material and the NOx catalyst. Thus, when the frequency of execution of the SOx poisoning regeneration processing is reduced, the fuel consumption amount related to the SOx poisoning regeneration processing is reduced. Further, when in the SOx poisoning regeneration processing the exhaust flows through both the NOx absorbing material and the NOx catalyst, the SV of the exhaust in the NOx absorbing material and the NOx catalyst is reduced, whereby the SOx purification ratio is improved.
In the exhaust emission purifying device for an internal combustion engine of the present invention, it is also possible to provide, in addition to the SOx poisoning regeneration processing means, are generation completion determination means for determining the SOx poisoning regeneration completion of the NOx absorbing material and the NOx catalyst. In this case, when it is determined by the regeneration completion determination means that the SOx poisoning regeneration of one of the NOx absorbing material and the NOx catalyst has been completed, the SOx poisoning regeneration means may control the exhaust flow switching means so as to prevent the exhaust from flowing to the substance on which the SOx poisoning regeneration has been completed.
The reason for this control is that, when the exhaust is continued to be supplied to the NOx absorbing material or the NOx catalyst on which SOx poisoning regeneration processing has been completed, the fuel component contained in the exhaust is burnt in the NOx absorbing material or the NOx catalyst to cause an unnecessary rise in the temperature of the NOx absorbing material or the NOx catalyst, thereby causing a heat deterioration in the NOx absorbing material or the NOx catalyst.
Further, when it is determined by the regeneration completion determination means that the SOx poisoning regeneration of one of the NOx absorbing material and the NOx catalyst has been completed, the SOx poisoning regeneration means may interrupt the SOx poisoning regeneration processing and cool the one on which the SOx poisoning regeneration has been completed, resuming the SOx poisoning regeneration processing solely on the one on which the SOx poisoning regeneration has not been completed yet after the completion of the cooling of the one on which the SOx poisoning regeneration has been completed.
In this case, the NOx absorbing material or the NOx catalyst on which the SOx poisoning regeneration has been completed is not left at a high temperature, whereby it is possible to further improve the durability of the NOx absorbing material and the NOx catalyst.
Next, in the exhaust emission purifying device for an internal combustion engine of the present invention, when the NOx catalyst consists of an occlusion reduction type NOx catalyst, it is possible to further provide an NOx absorption amount detection means for detecting the amount of nitrogen oxides (NOx) absorbed by the NOx absorbing material and the amount of nitrogen oxides (NOx) absorbed by the NOx catalyst.
When the NOx catalyst consists of an occlusion reduction type NOx catalyst, NOx are absorbed by the NOx catalyst on the same principle as that of the NOx absorbing material, so that it is necessary to release and purify the nitrogen oxides (NOx) absorbed by the NOx catalyst before the nitrogenoxide (NOx) absorbing capacity of the NOx catalyst is saturated. In view of this, in the exhaust emission purifying device of the present invention, there is provided an NOx absorption amount detection means capable of detecting the amount of nitrogen oxides (NOx) absorbed by the NOx catalyst in addition to the amount of nitrogen oxides (NOx) absorbed by the NOx absorbing material.
It is preferable that the NOx absorption amount detection means estimates the amount of nitrogen oxides (NOx) absorbed by each of the NOx absorbing material and the NOx catalyst on the basis of the amount of exhaust leaking from the exhaust flow switching means. In this case, it is possible to estimate the absorption amounts of nitrogen oxides (NOx) of the NOx absorbing material and the NOx catalyst with high accuracy, whereby it is possible to accurately set the execution time for releasing and purifying the nitrogen oxides (NOx).
In the exhaust emission purifying device for an internal combustion engine of the present invention, when it is necessary to control the exhaust flow switching means such that the exhaust flows through both the NOx absorbing material and the NOx catalyst, it is possible to further provide an NOx purifying means for controlling the exhaust flow switching means such that the exhaust flows through both the NOx absorbing material and the NOx catalyst after all the nitrogen oxides (NOx) absorbed by the NOx absorbing material and the NOx catalyst have been released and purified.
This means is provided for the case in which the NOx absorbing capacity of the NOx absorbing material is the same as the NOx absorbing capacity of the NOx catalyst. In this case, the nitrogen oxide (NOx) absorption amount of the NOx absorbing material and the nitrogen oxide (NOx) absorption amount of the NOx catalyst are set to zero before the exhaust flows through both the NOx absorbing material and the NOx catalyst, so that the time when the nitrogen oxide (NOx) absorbing capacity of the NOx absorbing material is saturated is the same as the time when the nitrogen oxide (NOx) absorbing capacity of the NOx catalyst is saturated. As a result, the nitrogen oxide (NOx) releasing/purifying processing for the NOx absorbing material is performed at the same time as the nitrogen oxide (NOx) releasing/purifying processing for the NOx catalyst, with the result that the execution frequency of the nitrogen oxide (NOx) releasing/purifying processing decreases, whereby it is possible to reduce the fuel consumption amount related to the NOx releasing/purifying processing.
On the other hand, when the nitrogen oxide (NOx) absorbing cap capacity of the NOx absorbing material is different from the nitrogen oxide (NOx) absorbing capacity of the NOx catalyst, all the nitrogen oxides (NOx) absorbed by the NOx absorbing material and the NOx catalyst are released and purified before the exhaust flow switching means is controlled such that the exhaust flows through both the NOx absorbing material and the NOx catalyst. And, when the exhaust flow switching means is controlled such that the exhaust flows through both the NOx absorbing material and the NOx catalyst, the NOx purifying means simultaneously releases and purifies the nitrogen oxides (NOx) absorbed by the NOx absorbing material and the nitrogen oxides (NOx) absorbed by the NOx catalyst, using, of the NOx absorbing material and the NOx catalyst, the one whose NOx absorbing capacity is less as a reference.
The exhaust emission purifying device for an internal combustion engine of the present invention is arranged in the exhaust passage of a lean-burn type internal combustion engine, and may comprise an NOx absorbing material which absorbs nitrogen oxides (NOx) when the air-fuel ratio of the exhaust flowing-in is lean and which releases the nitrogen oxides (NOx) it has absorbed when the oxygen concentration of the exhaust flowing-in is low, a bypass path branching off from the exhaust passage on the upstream side of the NOx absorbing material and allowing the exhaust to flow so as to bypass the NOx absorbing material, an exhaust flow switching means selectively switching the exhaust flow between the NOx absorbing material and the bypass path, an SOx absorbing material arranged in the exhaust passage on the upstream side of the exhaust flow switching means and adapted to absorb sulfur oxides (SOx) when the air-fuel ratio of the exhaust flowing-in is lean and to release the sulfur oxides (SOx) it has absorbed when the oxygen concentration of the exhaust flowing-in is low, and a NOx catalyst provided in the exhaust passage on the downstream side of the bypass path and adapted to purify the nitrogen oxides (NOx) when the air-fuel ratio of the exhaust is lean.